Although there is a long history of research on basic cognitive and motivational processes in ADHD and a long history of research on effective treatments for ADHD, there is almost a total absence of research relating basic processes to the impact of clinical interventions. Contemporary theories of ADHD emphasize a range of basic cognitive processes, including inhibitory control, working memory, and sustained attention, and basic motivational processes such as delay-related impulsivity (a tendency to choose small, immediate rewards over larger, delayed rewards). An important, yet untested, corollary of these models is that the primary treatments for ADHD - stimulants such as methylphenidate (MPH), behavioral treatment, and their combination - exert their effects by improving these basic processes. Thus, there are important gaps in our understanding of the extent to which the basic processes are sensitive to each of these treatments, and there are almost no data to address whether changes in basic cognitive and motivational processes actually account for treatment effects in clinical settings. The proposed research fills these gaps. First, it is the first study to concurrently examine the effects of both MPH and performance-based motivational incentives (i.e., monetary rewards, an analogue of behavioral treatment) on laboratory measures of inhibitory control, working memory, sustained attention, and delay-related impulsivity in children with ADHD (n=108). A group of age- and gender-matched controls (n=108) is included for comparison but not given MPH. We hypothesize that children with ADHD will exhibit impaired performance in the absence of motivational incentives, but that this pattern will be ameliorated in the presence of incentives for good performance. Second, among the children with ADHD, incentive will be crossed with doses of extended-release MPH (placebo, O.3., and 0.6 mg/kg) on a within-subjects basis to provide an initial examination of the separate and interactive effects of these analogue treatments on neurocognitive processing. Third, the proposed research will be the first to test the extent to which MPH effects on basic processes assessed in the lab actually mediate, or account for, individual differences in clinical response to MPH. To accomplish this aim, the participants with ADHD will undergo a 3-week, double blind school-based medication assessment to examine MPH effects on ADHD children's functioning in a natural setting. Thus, the proposed research will bridge basic and clinical research in ADHD and will provide critical initial tests of the hypothesis that changes in basic cognitive and motivational processes are the mechanisms by which treatments for ADHD work. This work will pave the way for a new generation of translational research and theory in ADHD.